1. Field of the Invention
The invention relates to a wire tensioning device, more specifically a static wire tensioner, particularly for use on coil winding machines.
It is known that, in machines for winding metal wire, more particularly enamelled copper wire to form coils for use in an electric and/or electronic field, it is necessary to dispose a tensioner-adapted to keep the tension of the wire being supplied at a substantially constant value--between the supply spool and the wire guide winding the wire.
The wire is usually wound in the axial direction onto the coil, in a substantially free manner, so that a tensioner is essential for tensioning the wire in order to obtain properly wound coils.
The wire tensioning devices at present in use are mainly of two kinds: static and dynamic.
Preferance is frequently given to dynamic tensioners, which have the main advantage of recovering any slack wire released during the operating phases, and also suffer from very little wear.
On the other hand, it is known that dynamic tensioners are completely unsuited for tensioning wires which have to be supplied at high speed and discontinuously, e.g. in the case where square or rectangular coils are being wound, since they may cause irregular winding and damage or break the wire.
As known, these drawbacks are obviated in static tensioners, which usually comprise means for clamping the wire at a set pressure.
2. Description of the Prior Art
The most well-known and widely-used static wire tensioners comprise a pair of oppositely-disposed shoes having, for example a felt, leather or similar surface, between which the wire is clamped at a set pressure. These tensioners are very accurate and efficient when new, but after a relatively short period in operation the wire, which runs in close contact with the shoe surface, forms a grrove thereon, so that the tension cannot be efficiently regulated.
Another and more recent kind of static wire tensioner comprises a bundle of glass fibres having an axis along which the wire moves. The bundle is subjected to torsion around the aforementioned axis with a varying amount of force, thus clamping the metal wire to a varying extent. However, also the latter kind of tensioners soon present the same drawbacks i.e. the metal wire forms a groove as it runs inside the bundle, so that the tension cannot be efficiently regulated.
Other static wire tensioning devices have been studied and proposed but they all have the same drawbacks owing to the relatively rapid wear of the material along which the wire runs.
It is known that attempts have been made to use two oppositely-disposed shoes made of metal and consequently less subject to wear. This method, however, though having a certain advantage in respect of wear, has nevertheless the following drawbacks.
Firstly, it is difficulat to keep the metal shoes exactly parallel, irrespective of the diameter of the wire to be tensioned. In fact, the shoes have to be kept perfectly parallel since otherwise, particularly in the case of thinner wires, there is a risk that the clamping pressure of the shoes may be released at a point of direct contact between the shoes, rather than on the wire to be tensioned.
It has also been found that, at least under certain conditions, the contact under pressure between copper wire--enamelled if required--and metal shoes, results in damage to the shoes as well as the wire, and such damage cannot be neglected.